Presentation
Right flank pain.
Patient Data
No renal stone seen in the right kidney.
Shadowing calculus in the interpolar right kidney, measured at 9 x 15 mm.
There is a faintly hyperattenuating area in the right interpolar kidney at a medullary pyramid tip.
On these annotated images, the arrow points to a faintly hyperattenuating interpolar stone.
Case Discussion
Relatively radiolucent stones are a potential blind spot for low dose CT. It was tempting to read the low dose CT as "no renal stone", but when comparing with prior studies there had been a shadowing calculus at the interpolar kidney. When looking at the most recent CT available before that, there was a faintly hyperattenuating area at an interpolar kidney medullary pyramid tip.
Although uncommon, this suggested that the patient may have a radiolucent stone / matrix / medication-induced stone. On review of the patient's chart he had been administered antiretrovirals (dolutegravir/rilpivirine), and this class of medications has been associated with matrix stones.